Public `setModifierManager` is not documented

The [merged RFC](https://github.com/emberjs/rfcs/blob/f0a9b20d898013994b01670a35f868c229b2771b/text/0373-Element-Modifier-Managers.md#determining-which-modifier-manager-to-use) specifies a public import `setModifierManager`:

original RFC text

> Ember will provide a new API to assign the modifier manager for a element modifier
class:

>
> ```js
> // my-app/app/modifier/foo.js
>
> import EmberObject from '@ember/object';
> import { createManager } from './basic-manager';
> import { setModifierManager } from '@ember/modifier';
>
> export default setModifierManager(createManager, EmberObject.extend({
> // ...
> }));
> ```
>
> ```js
> // my-app/app/modifier/basic-manager.js
>
> // ...
>
> export function createManager(owner) {
> return new BasicManager(owner);
> }
> ```
>
> `setModifierManager` takes two parameters. The first parameter is a function that takes an `Owner` and returns an instance of a manager. The second parameter is the base class that applications would extend from.

Currently, `setModifierManager` is undocumented:

https://github.com/emberjs/ember.js/blob/ef3688b0986d06f4902166d8433a0441ae70af46/packages/%40ember/-internals/glimmer/lib/utils/custom-modifier-manager.ts#L1-L5

We should document it!